FIG. 1 shows a typical audio system. In the example considered, the system comprises an audio signal generator 10, such as a radio, CD or MP3 player, generating an analog audio signal AS to be sent to at least one speaker 30.
In the example considered, an audio amplifier 20 is interposed between the audio signal generator 10 and the speaker 30, which is configured to generate an amplified audio signal AAS by amplifying the analog audio signal AS provided by the audio signal generator 10.
For example, FIG. 2 shows a possible implementation of a so called class D audio amplifier 20.
Specifically, in the example considered, the audio amplifier 20 comprises a waveform generator 202 generating a periodic triangular waveform signal TS, having typically a frequency between 250 kHz and 2.5 MHz. The triangular waveform signal TS is sent together with the audio signal AS to a comparator 204, which compares the audio signal AS with the triangular waveform signal TS thereby generating a square wave signal DS, whose duty-cycle varies as a function of the amplitude of the audio signal AS.
The square wave signal DS is then amplified by an amplifier stage 206, usually comprising two electronic switches, thereby generating an amplified square wave signal ADS. Finally, the amplified square wave signal ADS is sent to a low-pass or bandpass filter 208, which removes at least the high-frequency spectrum from the signal amplified square wave signal ADS, thereby generating the amplified audio signal AAS which is proportional to the original audio signal AS. The operation of such a class D amplifier is well known to those skilled in the art, rendering a more detailed description herein superfluous.
Substantially, the class D amplifier is based on the fact that the switching frequency of the amplifier 20 is significantly higher than the usual audio band (between 20 Hz and 20 kHz) and accordingly the high switching frequency may be filtered with the filter stage 208, thereby reconstructing the profile of the original audio signal AS.
In the context of digital audio data, the signal generator 10 may comprise an analog-to-digital converter for generating the signal AS or the signal generator 10 may provide directly the digital signal DS.
In any case, substantially similar low-pass or bandpass filters 208 are provided in most audio amplifier circuits and/or may be integrated also within the speaker 30
Accordingly, generally, an audio system may be modelled as shown in FIG. 3.
In the example considered, the system comprises a generic signal generator, such as a voltage source VS, a filter stage 208 configured to filter the signal provided by the signal generator, and a speaker 30 which reproduces the signal provided by the filter stage 208. Accordingly, in general, the filter stage 208 comprises two input terminals for receiving the signal provided by the signal generator VS and two output terminals for providing the filtered signal Vout.
For example, in the example considered, the filter 208 is a LC filter. Specifically, in the example considered, the first input terminal is connected to the first output terminal via an inductor L, and the second input terminal and the second output terminal are short circuited and represent a ground GND. Finally, a capacitance C is connected in parallel with the output, i.e., between the output terminals.
For service and/or safety reasons, audio systems are often provided with a diagnostic system configured to detect the connection status of the load, i.e., the speaker(s) 30. For example, preferably the diagnostic circuit should be able to detect whether the speaker 30 is damaged, e.g., short circuited or disconnected. For example, a short circuit between the output terminals of the audio amplifier 20 may cause a high current flow, which may overheat and/or damage the audio amplifier 20.
Usually, the diagnostic circuit operates when the signal generator 10 is deactivated. In this case, a given audio test signal with given frequency characteristics is applied to the audio amplifier 20 and the output current Iout is monitored in order to determine whether the speaker 30 is connected to the audio amplifier 20. For example, such a diagnostic operation may be performed when the audio system is switched on or during a test cycle.
However, the above solution is unable to detect the detachment of the speaker, while the audio signal generator 10 provides the signal AS to the audio amplifier. In fact, the characteristics of the audio signal, for example music, are usually not stable. Moreover, the impedance of the speaker 30 does not represent a pure resistance but may be modelled with a RLC circuit. Finally, a speaker 30 subject to vibrations (as usual in the automotive field) may even generate a current. For these reasons, a simple monitoring of the output current Iout is likely to generate false detections.